I have topped off (by accident the first time) with the juice concentrate with out diluting it and while the wine is still a bit "hot" the flavor and body was much better, and it took another 6 weeks of mild fermenting-- I think that the extra aging will be worth it.
My favorite of the old orchard concentrates is the apple raspberry, and I might start another with the apple cherry, and I start off with three cans (12oz) per gallon and then dilute them with apple juice.
Using the juices and concentrates for one gallon trial batches are giving me the confidence to work my way up to the fruits from the back yard this coming summer.